1. Field of the Invention
The present invention relates generally to web printing presses and more particularly to a device and method for calibrating a web tension transducer.
2. Background Information
Web printing presses print a continuous web of material, such as paper. In order to do so, tension measurements of the web can be critical to the operation of a web press. Different paper types produce differing amounts of tension in the web proportional to the speed of the web and the paper modulus.
One way to measure web tension is using analog tension transducers. However, existing tension transducers are analog devices and require frequent calibration, so that an electrical signal produced by the transducer can be correlated properly to actual web tensions. The current calibration procedure is performed when the web is not operating by a manual measurement of signals produced by the transducer. This method is time consuming and the accuracy is a function of human performance. Moreover, the calibration must be conducted when the press is not in operation. This can result in lost production due to the downtime required for calibration of the tension transducer and poor press performance resulting from the erroneous tension information supplied by uncalibrated tension transducers.
An object of the present invention is to provide a device and method for providing a process of automatic tension transducer calibration. An additional or alternative object of the present invention is to provide a method for calculating the amount of tension in a paper at a discrete speed.
The present invention provides an apparatus for calibrating a tension transducer for a web including a pair of nip rollers and a motor for driving at least one of the pair of nip rollers, the motor having at least one roller output being a function of at least one of a motor torque and a motor velocity. A web tension transducer roll is located upstream of the nip rollers, the web tension transducer having a tension output being a function of the actual web tension of the web. A processor receives the at least one roller output and the tension output and calculates a tension correction value as a function of the at least one roller output and the tension output.
The tension transducer thus can be calibrated without manual interference, using the tension correction value.
The tension correction value further may be a function of a predicted web tension, which may be determined using a radius of the transducer roll, or a predicted change in web tension, which may be determined using a paper modulus.
Preferably, the at least one roller output includes both a motor torque output and a motor velocity output.
The processor then outputs a calibrated web tension value as a function of the tension output and the tension correction value, and may also calculates a breakaway torque of the motor as well as a correction offset of the tension output.
The processor can calculate a correction factor, which can be the tension correction value, as a function of the at least one roller output and the predicted web tension or predicted change in web tension.
The present invention also provides a method for calibrating a tension transducer for a web comprising the steps of:
running the web over a tension transducer and through nip rollers, the nip rollers being run by a motor;
varying a characteristic of the motor and reading actual reference signals from the transducer as the characteristic is varied; and
calculating at least one correction value as a function of the actual reference signals.
The method may further comprise reading a first signal from the tension transducer at a known web tension, preferably zero, so as to determine a correction offset, and may further include determining a breakaway torque of the motor. The method may include repeating the step of determining a breakaway torque a plurality of times, so as to calculate an average value for the breakaway torque.
The characteristic preferably is a motor torque and the varying step occurs between the breakaway torque and a predetermined torque. The actual reference signals then may be linearized so as to form an actual tension versus torque line.
Alternatively, the characteristic may be a motor velocity and the varying step may include changing a motor velocity with respect to a nominal velocity. The at least one correction value preferably then is further a function of a known paper modulus.
The actual reference signals are compared to a predicted data set, the at least one correction value preferably including a correction factor being a function of the actual reference signals and the predicted data set.
The method may further comprise calculating a calibrated web tension output as a function of actual signals from the tension transducer and the least one correction value.
The characteristic preferably is a motor torque value or a motor velocity value.
The calculating step may further include calculating a ratio of a predicted data angle over a measured data angle, so as to form a correction factor.